spanning-tree optimization

I have a four switches layer-2 setup running RSTP. For one of the vlan spanning tree is not optimized and root switch is selected be default (lowest MAC) process.

1. What will happen if I configure the "spanning-tree primary" for a VLAN on the switch that is already the root for this VLAN? COnsidering that I am using RSTP would there be any impact to the user?

2. What will happen if I configure a differnent switch as "spanning-tree primary"? Would it take more time to converge? User impact?

3. A switchport is in default "dynamic desirable" mode. It is configured for access vlan "switchport access vlan .." and is supposed to be in access mode but does not have the mode defined. If I go ahead and apply the command "switchport mode access " would it have any impact?

1) spanning-tree <> root primary is just a macro that can be used to set the priority of a particular bridge. If, as a result of using this macro, the switch has the best bridge ID in the network, it will become root. Else it won't. You're not guaranteed that the bridge will become root as a result of using this macro, if this is what you are wondering.

2) No impact on speed. Same answer as above. In fact, the spanning-tree root primary is rather confusing when it's a matter of selecting the root. I think it's much simpler to put a priority manually. Question of taste probably, but without the root macro, you would not be confused into wondering what it does exactly;-)

3) No relation to STP right? The port can be access or trunk. You can hardcode the port as access or trunk. You can let DTP negotiate the final mode with a peer. In that case, you have several options. I'm not sure exactly what kind of information you are looking for in fact, sorry about that.